The invention relates to an electronic flash unit comprising a flash tube, a storage capacitor dischargeable through the flash tube, a flash ranging or measuring device to record the quantity of light emitted by the flash tube and reflected by the photographic object (i.e., the subject being photographed), a pulse generator circuit capable of being switched on or rendered operative when an adjustable threshold value of the recorded or measured quantity of light is reached, and a device to quench the flash tube, which device can be switched on by means of a pulse generated by the pulse generator circuit. Flash apparatus of this general type is well known in the art, and the present invention deals with improvements therein.
With such electronic flash units, it is often desired that the photographer should be able to keep a constant check on whether the flash ranging or measuring device, the pulse generator circuit, and the flash tube quenching device, (usually collectively termed a "computer") have become operative when the photograph was taken.
In one known electronic flash unit of this kind, the quenching device for the flash tube contains a gas-filled short-circuiting tube or thyratron, which short-circuits the flash tube at the moment when the thyratron ignites. Such a thyratron emits, in its current-carrying state, a visible light, which is utilized for checking the operation of the computer. To accomplish this, a light guide bar or conductor extends from the thyratron to a window in the surface of the casing or housing of the unit. Thus the photographer can observe the lighting-up of the thyratron during the short-circuiting of the flash tube. Since the lighting-up time of such a thyratron is only a few milliseconds, a fluorescent, luminescent or phosphorescent disk is provided at or near the window at the end of the light guide bar. Due to the afterglow of this disk, it is possible for the photographer to see, even after completion of the flash process, whether the blocking device has become operative during the preceding flash radiation.
This display of the operation of the computer has a decisive disadvantage. Although the afterglow of the fluorescent disk indicates to the operator that the computer has become operative once, yet when there is a plurality of flash releases in rapid succession the operator cannot tell whether the computer has worked in each individual case. Therefore, he is also unable to find out when the computer may have failed for the first time. Moreover, such an indicating device comprising a light guide bar is relatively expensive and, due to the necessary space requirement, it cannot be used in so-called compact flash units, in which all the structural elements are housed in a very narrow space.
Furthermore, such an indicating device requires that the quenching device of the flash tube includes a structural element which emits a visible light. However, in specific cases of application, particularly in order to achieve high-speed flash sequences, such a gas-filled switching tube, which can only be quenched through the voltage breaking down at its anode-cathode path, has to be replaced by a semi-conductor element, preferably a thyristor, which can be forcibly blocked in spite of the fact that voltage is applied to its anode-cathode path. However, such a thyristor does not emit any visible light, so that in these cases there is no basis whatsoever for an indicating device of the kind above mentioned.
The task underlying the present invention, therefore, is to provide an electronic flash unit of the general type or kind mentioned at the beginning, with a device indicating the operation of the flash unit computer, which device avoids the disadvantages described above, and does not depend on the presence of a switching element emitting a visible light and which will give a signal that is constant in its intensity over a selectable period.
According to the present invention, this problem is solved by providing a light-emitting diode, which is connected to a direct current source via an electronic switch which closes in response to a signal derived from the pulse generator circuit or the quenching device and which opens automatically after an adjustable closing period.